What is the ideal mast bend needed for the IOM rig (my view which may or may not be correct):
Light wind
A straight mast or one with a slight bend depending on main luff curve. Slack shrouds and soft jib luff so it is nice and full and allows the jib boom to rotate freely.
Medium wind
Max shroud tension, tight jib luff, mast bend to suit sail chord but likely even bend throughout
Heavy wind
As medium but be able to flatten everything and blade out top of main to depower.
Unfortunately to achieve the above you would need two masts. In light weather you would need little prebend. In medium to heavy weather, you need max prebend so we have to make a compromise.
I used 12-15 mm of prebend on the britpop according to the Britpop bible which worked nicely and gave me complete control of the rig. The thing to remember when you put prebend into the mast is that you can never get the bend right to the end of the mast. The mast tube normally requires a length to be cut off to bring it to 1.7m. Cut off the unbent end and you will have prebend up to approx 150mm from the top of the mast (depending on your bend machine) and then rest will be straight. Cut off the prebent end and you will have prebend right to the top. I favour the latter approach which will be on my next mast although it would be nice to have the prebend taper to the top of the mast. The only challenge with this is you want to have a straght mast at the top when under load which does not always happen when you prebend to the top of the spar.
How do the forces on the mast work together
Have a look at the picture which is drawn to scale. The force couple that dictates jib luff tension is created by the back stay, the jib luff along with the hounds which provide mast bend resistance and to a certain extent the shrouds. To see the effect of the shrouds on the mast, just squeeze the shrouds together and watch the mast invert. I currently have 16mm prebend down to 800mm but the top 150mm of the mast is straight. I can hold the whole mast straight in any weather but the top (from jib attachment to top) part of the mast bends aft which reduces jib luff tension and flattens the top of the sail opening the leech.
My next move to eliminate this top bend is to try Zvonko style shrouds (on the current mast), to see if that straightens the top and increase jib luff tension.
To set this up, the shrouds go through an eye at the hounds and then run to a hook a few inches above the jib band (picture below). The eye at the hounds is adjustable so I will be able to vary hound height. This change should make for a stiffer mast in the top third so I will need to depower using lower or an even mast bend.
I am looking forward to seeing the unintended consequences of this change. It will mean a tiny bit more weight and windage up the mast, but it could mean I could carry less prebend and a softer rig for better light weather performance and more forestay tension in a breeze.
The unintended consequences
I sailed at Gosport in gusty conditions this morning with the new shrouds I set the boat up with the hound at the Britpop/Alternative height, tight shrouds and lots of kicker and the boat was hard to sail with weather helm. The rig would not give in the gusts and the mast too straight. As the morning went on, I eased the kicker and the shrouds (to keep the leeward one just tight) so the mast could try and bend over its full length and the boat came alive. The middle of the mast tries to invert with any kind of shroud tension so that would need looking at. The spreaders are bent aft a few degrees.
When I got home, I solved the problem of the inversion. Just move the hound point further up the mast, reducing the inversion and creates a lovely even bend throughout the mast. So now I can have a straight mast, bend at the top, a fair curve and anything in between. Job done.
There is lots to play with here but I love the flexibility the set up delivers. Here is the picture of the hound to shroud termination. Ignore the unfinished bits of string. They will be cut to length. The shrouds are terminated onto a hook..
Regarding spreaders, I have set mine as short as 45mm and as long as 55mm. The 45mm made it hard to control the sideways bend of the mast even with very tight shrouds. It was good for depowering but did not allow me to point high enough upwind. With the 55mm the rig was just too stiff so I have settled on 50mm which provides good support for the rig but allows the mast to flex a bit in gusts. Remember I am only talking about a rig on the Proteus design. You would have to work out what works for your design.
So the more I think about rigs, the more options I have to consider and the question in my mind is how do I want the rig to work when I am sailing.
e.g Do I want the top mast to fall away to depower in gusts or do I want the middle of the mast to work slight to flatten the main and open the leech. With my new setup, if I set the hounds high the middle part of the mast can work in the gusts. If I lower the hound, then I stiffen the upper middle of the mast and the only method of depowering is for the top to fall away. Of course there are options in between the two extreme settings so the only way to find out is to test on the water. My preference is to let the middle of the mast work as I can easily control that.
Maybe I will end up fixing the shrouds higher up similar to the V12 setup. It all depends how the boat feels and behaves.
I think Zvonko made a great discovery here and I applaud his innovation skills although it has to be said Graham Bantock suggests running the shrouds up the mast but inside rather than outside.
For the testing I fix most of the variables and just change one thing. The weather is looking good this Saturday.
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